This study deals with patch loaded web plates, modeling themselves upon girders erected with launching method. To stiffen such web plates, a horizontal stiffener is generally used because a load bearing stiffener cannot be arranged to such web plates. However, a horizontal stiffener is originaly aimed to restrain not the buckling due to the patch load but the buckling due to in-plane bending. Thus discussion on the effectiveness of a stiffener is required.In this study, an elasto-plastic large deflection FEM program is used for the analysis, and existence, location and stiffness of a horizontal stiffener, magnitude of in-plane bending moment and so on are adopted as parameters.Through the analysis, followings are found;1. Strength of patch loaded web plates are reduced considerably by in-plane bending moment.2. On a case of no or small in-plane moment, the ultimate strength of web plates with a horizontal stiffener is sometimes smaller than the strength of those with no stiffener.3. Even on above case, a horizontal stiffener increases buckling strength of web plates.4. The collapse mode of web plates are classified into following four types according to the parameters; (1) Collapse by the mechanism having two yield lines. (2) Collapse by yield lines and in-plane compression yielding at the bottom of the web plate. (3) Collapse by the in-plane compression at the bottom of the web plate. (4) Collapse by yielding of the upper and lower flanges due to the girder bending.